1. Field of the invention
This invention relates to a silicon carbide semiconductor device, and more particularly, it relates to a silicon carbide semiconductor device having at least one ohmic electrode in contact with a silicon carbide single-crystal layer. It also relates to a method of producing such a silicon carbide semiconductor device.
2. Description of the prior art
Silicon carbide (SiC) is a semiconductor material with a wide band gap of 2.2 to 3.3 electron-volts (eV) depending on its crystal structure. Furthermore, silicon carbide is thermally, chemically, and mechanically quite stable, and can be of either p-type or n-type with high stability, which is a rare case with wide-gap semiconductor materials. Therefore, semiconductor devices having a silicon carbide single-crystal layer and at least one ohmic electrode formed in contact with the silicon carbide single-crystal layer for electrical connection to external circuits are expected to have wide applications in the field of electronics, such as optoelectric transducers and semiconductor devices which can be used under severe conditions of high temperature, high output drive, and radiation exposure.
Recently, the inventors have invented several silicon carbide semiconductor devices, and filed for example, Japanese Patent Applications No. 1-221207, No. 1-145617 and No. 1-145618, which correspond to U.S. patent applications Ser. No. 574,487 (filed on Aug. 28, 1990), No. 533,573 (filed on Jun. 5, 1990) and No. 534,046 (Jun. 6, 1990), respectively. These silicon carbide semiconductors have aluminum (Al) wiring electrodes formed on a silicon carbide single-crystal layer.
Other examples of the ohmic electrode formed on a silicon carbide single-crystal layer include those obtained by vacuum deposition of nickel (Ni) in the case of n-type silicon carbide single-crystal layers or a eutectic material of aluminum (Al) and silicon (Si) in the case of p-type silicon carbide single-crystal layers on the surface thereof, and then alloying at a high temperature of around 1100.degree. C. (see, e.g., "Breakdown Field in Vapor-Grown Silicon Carbide P-N Junctions," Journal of Applied Physics, Vol. 48, No. 11, pp. 4831-4833(1977)). Also included are titanium electrodes formed by vacuum deposition or the like (see, e.g., Japanese Laid-open Patent Publication No. 62-71271 which corresponds to U.S. patent application Ser. No. 910,482 filed on Sept. 23, 1986).
However, when ohmic electrodes as mentioned above are used, the step of alloying at a high temperature of about 1100.degree. C. causes the following problems: (1) the cohesion of electrode metal occurs which makes it difficult to form a uniform ohmic electrode; and (2) stress is applied by the cohesion of electrode metal to the silicon carbide single-crystal layer in contact therewith, resulting in a reduction of crystallinity due to increased crystal deformation and dislocation. Moreover, vacuum deposition of titanium introduces the following problems: (1) it is difficult to form thick films for use as an electrode because of its high melting point of 1720.degree. C.; and (2) it is necessary to form another deposited layer of aluminum or the like because of difficulty in direct connection with external wirings.